The present invention relates to sheet feeding and more particularly to a vacuum corrugation feeder with the capability of detecting the degree of curl in sheets.
With the advent of high speed xerographic copy reproduction machines where:in copies can be produce at a rate in excess of three thousand copies per hour, the need for a document and sheet feeder to, for example, feed documents to the platen of a copier in rapid succession in a reliable and dependable manner in order to utilize the full capabilities of the copier. A number of document handlers are currently available to fill that need. These document handlers must operate flawlessly to virtually eliminate the risk of damaging the original document and generate minimum machine shutdowns due to misfeeds or document multifeeds. It is in the initial separation of the individual documents from the document stack where the greatest number of problems occur which, in some cases, can be due to upcurl and downcurled in documents.
Since the documents must be handled gently but positively to assure separation without damage through a number of cycles, a number of solutions halve been suggested such as friction rolls or belts used for fairly positive document feeding in conjunction with a retard belt, pad, or roll to prevent multifeeds. Vacuum separators such a sniffer tubes, rocker type vacuum rolls, or vacuum feed belts have also been utilized.
While the friction roll-retard systems are very positive, the action of the retard member if it acts upon the printed face can cause smearing or partial erasure of the printed material on the document. With single sided documents, this does not present a problem as the separator can be designed so that the retard mechanism acts upon the underside of the document. However, with documents printed on both sides, there is no way to avoid the problem. Additionally, the reliable operation of friction retard feeders is highly dependent on the relative frictional properties of the paper being handled. This cannot be controlled in a document feeder.
Various approaches have been highly successful in answering the above problems, for example U.S. Pat. No. 4,305,576 discloses a typical vacuum separating and feeding system wherein a plurality of friction belts is arranged to run over a vacuum plenum placed at the bottom of a sheet supply tray which has a "U" shaped pocket formed in it. The pocket serves to provide space for the bottom sheet to be captured by the vacuum feed belt assembly, to provide an air seal between the bottom sheet and the edges of the pocket and to provide a high pressure seal between the bottom sheet and the remainder of the stack. This high pressure seal is achieved by supporting a major portion of the stack weight on the edge regions of the pocket. However, this "U" shaped configuration was found to not permit deformation of the sheet in a geometrically developable shape which results in a reduction in the degree of levitation of the sheet stack. The bottom sheet vacuum corrugation feeder in U.S. Pat. No. 4,411,417 answered this problem by including a differently designed stack support tray that has a planar base portion defining a base plane, the front of the base portion having a opening within which the bottom sheet separator is positioned. The tray also includes two sloping planar side wings, one at each side of the opening in the base portion. The sloping planar side wings are angled upward from the base plane and are angled outward from front to rear of the tray and intersect the base plane such that the intersection at the rear of the tray is in the approximate location of the rear corners of a rectangle the size of a sheet to be fed and the intersection of the planar wings and the base plane at the front of the tray is approximately midway between the front corners of a sheet to be fed and the center line of a sheet to be fed.
Even though the above two document handlers in U.S. Pat Nos. 4,305,576 and 4,411,417 have been highly successful, an improvement of the document feeder in U.S. Pat No 4,411,417 is still needed because of the presence of unpredictable curl in sheets which has been known to cause problems throughout the xerographic process. In present day intelligent paper handling devices, it is useful to know as much as possible about the qualities of the: paper which is passing through a machine. Various parameters such as timing, airflow, normal force and velocity can be adjusted to compensate for the presence of detectable levels of input stress, such as, sheet curl to ensure that the paper will arrive safely and in time to meet the interface requirements of the various subsystems of the machine. Copy quality parameters, such as, transfer or detack current, as well as, fuser and decurler settings can also benefit from this information.